The present invention relates generally to drying of coated films and, more particularly, to drying methods and apparatus for minimizing mottle and creating a uniform drying atmosphere.
One of the most common defects associated with organic solvent coatings is mottle. Direct impingement air can cause mottle by disturbing the coating. Also, the heat transfer uniformity is critical. Local variations in heat transfer can show up as mottle. Even if coatings are allowed to dry without direct air impingement, the shear forces caused by the web moving through still air can cause mottle. This will limit the speed at which a product can be manufactured. The occurrence of mottle is often cited as the single greatest limitation to productivity improvement in the drying of coated webs. In order to produce acceptable coatings, web speeds are often reduced significantly from what the machine is capable of coating and drying.
Mottle patterns can range from random and blotchy to xe2x80x9cliney-streakyxe2x80x9d depending on the coating and process conditions. Typically, in photographic film and paper, mottle becomes more severe and oriented in the direction of web travel as web speed is increased. Sensitive products can be limited to web speeds of around 150 feet per minute (fpm). Coatings can be made to be more robust to mottle by increasing the viscosity of the solutions and decreasing the wet thickness of the coating (concentrating the solution) such as described in Miller, C. A. and Neogi, P.; xe2x80x9cInterfacial Phenomenaxe2x80x9d; Marcel Decker; 1995 but, this is not always possible because of coatability or solution stability concerns.
When the coating solutions cannot be made to be robust to mottle, disturbances to the coating created in the coating and drying machine must be minimized in order to produce acceptable coatings. One of the most important disturbances is air. Air can directly disturb a wet coating if the pressure or shear forces are great enough (Gutoff, E. B. and Cohen, D.C.; xe2x80x9cModern Coating and Drying Technologyxe2x80x9d; J. Wiley and Sons; p. 289; 1995). Even if the pressure and shear forces are not great enough to blow the coating around, non-uniformities in the air velocity impinging on the coating can cause surface tension driven flow. Surface tension driven flow arises as a result of variations in concentration and temperature along the surface of the coating. Non-uniform air flow can cause local variations in heat and mass transfer rates which in turn cause concentration and temperature variations.
In the last several years there have been only a limited number of published reports on the reduction of mottle by controlling air flow in a solvent coating machine. U.S. Pat. No. 4,365,423 to Arter et al. describes using two-layer screens very close to the coating to protect it from air disturbances and to raise the local solvent concentrations in the gas. U.S. Pat. No. 4,999,927 discloses another apparatus and method for drying a liquid layer that has been applied to a carrier material moving through a drying zone and which contains both vaporizable solvent components and non-vaporizable components. Dying gas flows essentially parallel to and in the direction of the carrier material and is accelerated within the drying zone in the direction of flow. In this manner, laminar flow of the boundary layer of the drying gas adjacent to the liquid layer on the carrier material is maintained. By avoiding turbulent air flow, mottle is reduced. U.S. Pat. No. 5,105,562 to Hella et al. describes a ventilating and impinging air bar assembly primarily for improved conveyance. However, this design relies on direct front side air impingement which is, in general, not desirable from the standpoint of minimizing mottle.
U.S. Pat. No. 4,894,927 describes a process for drying a moving web coated with a coating composition containing a flammable organic solvent. The web is passed through a closed-type oven filled with an inert gas and planar heaters on top and bottom of the web. The flow of drying gas is parallel but counter-current to the direction of web movement. The coating surface is reported to be barely affected by movement of the inert drying gases due to the small amounts of gas required. No discussion of the criticality of the gas flow system or of the need to prevent mottle is given.
Generally the drying of coated webs is accomplished by direct impingement of air from a nozzle wherein the air is supplied perpendicular to the place of the coated web. Using this technique, mottle occurs in the coating.
U.S. Pat. No. 1,776,609 to Andrews discloses a web drying apparatus that consists of nozzles which discharge heated air onto a deflector member. The air is discharged in the direction of the web and the discharge velocity is high to provide a large heat transfer. There is no mention of mottle control or matching of air velocity to web velocity.
U.S. Pat. No. 5,105,562 to Hella et al discloses a web drying apparatus which consists of a direct impingement air bar discharging air against the coated surface and a dilution air bar mounted on both sides of the impingement bar. This configuration provides both parallel (to the web travel) air flow and counter (to the web travel) air flow. The direct impingement and dilution bars are supplied air independently of each other. There is no mention of trying to match the air velocity to web velocity to control coating mottle.
U.S. Pat. No. 6,018,886 to Bell et al. teaches the use of curved nozzles to deliver air uniformly across the web width in the drying section. The air is delivered substantially parallel to the substrate surface and in the direction of the substrate movement. The goal was to deliver air at approximately the same velocity as the substrate speed. Although the average air velocity through the length of the drying section is close to the speed of the coated substrate through the drying section, the local air velocity can vary considerably along the length thereof. Even localized variability in the air velocity can lead to mottle.
It is an object of the invention to provide an apparatus and a method for drying coated webs without causing mottle.
It is a further object of the invention to substantially eliminate the shear effects that can be created by the coated web as it passes through the air in a dryer.
Briefly stated, the foregoing and other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by providing an improved configuration of the nozzles with the dryer over that taught in Bell et al. A minimum distance between the nozzle and the substrate surface is required to prevent periodic low and high regions of air velocity. An attachment plate is required to capture the air from the preceding nozzle and to further prevent the regions of low air velocity. This attachment plate is substantially parallel to the substrate surface.